sip_session: use SCSessionGotStreamUpdate to update prompt

darcs-hash:20090326082240-b2873-baee1b1ded58dfa1668c76c587bed5ccbeadfbe2
sip_session: in case there were no streams at all, print reasonable message

darcs-hash:20090326082336-b2873-220c03b79797c8540ca8c576aa34de79b2cb8240